1. Field of the Invention
The present invention relates to a printer that prints an image on a print medium by transferring ink from a ink ribbon to the print media and, more specifically, to a printer that comprises a platen roller of which the circumference is smaller than the length of the print medium, a capstan roller which clamps and transports the print medium, and a pinch roller.
2. Description of the Related Art
The related art will be explained with reference to FIGS. 1,2 and 3.
Generally, a color printer uses three-colored or four-colored ribbon, which contains yellow (Y), magenta (M), and cyan (C), or black (K) added to them, and obtains a color image by printing each of the above colors on a print medium. Basically, this kind of printer comprises a platen roller, a thermal head that is pressed to the platen roller, an ink ribbon and a print medium, both of them are transported between the platen roller and the thermal head to produce a color image.
FIG. 1 shows a schematic diagram of a color printer 100 of prior art having a platen roller 102 of which circumference is longer than a length of a print medium 101.
The platen roller 102 is driven by a platen motor (not shown) and can rotate around a shaft 103 in a direction of arrow. A clamper 104 lies across the surface of the platen roller 102, and cramps a leading edge of the print medium 101 that is fed by a media feeder (not shown) and wraps the print medium 101 around the platen roller 102. The platen motor drives the platen roller 102, which rotates in the allowed direction. An ink ribbon 105 is fed from a supply roll 106 and to a take-up roll 107. A thermal head 108 is depressed to the platen roller 102. Drivers (not shown) selectively energize a plurality of thermal elements 108a of the thermal head 108, and transfer ink from the ribbon 105 to the print medium 101. This is the way of this system for printing an image.
To obtain a color image, the ink ribbon 105 requires to be a four-colored ribbon, as shown in FIG. 2. Repeating an aforementioned process four times, a color image is produced.
FIG. 3 shows a schematic diagram of another color printer 200 of prior art that is disclosed in the Japanese Laid-Open Patent Publication H3-39271.
In the above mentioned color printer 200, a cylindrical sheet guide 202 is installed and has a longer circumference than the length of a print medium 201 in a direction of thereof. In the opening part 202a of the sheet guide 202, there is a platen roller 203 that has a shorter circumference than the length of a print medium 201, and can rotate. In both sides of the sheet guide 202, a pair of arms 204 is rotatably installed in a drive shaft 205. At the end of arms 204, a grip 204a is fitted and cramps the print medium 201.
The grip 204a cramps the end of the print medium 201 that is fed by a sheet feeder (not shown), and rotates along surface of the sheet guide 202 together with the arms 204 in the arrow direction. A ribbon film 206 is fed from a supply roll 207 to a take-up roll 208. The platen roller 203 rotates as the print medium 201 moves. A thermal head 209 is pressed to the platen roller 203. Drivers (not shown) selectively energize a plurality of thermal elements 209a of a thermal head 209 in response to the image data, and transfers ink onto the print medium 201 and finally produces an image. In the case of a four-colored ink ribbon 206, a color image is printed by repeating the rotation of both the print medium 201 and the arms 204 four times.
The diameter of the platen roller 102 has to be made large, as the circumference of the platen roller 102 of the color printer 100 is longer than the length of the print medium 101. It is difficult to make the large diameter platen roller 102, because it is difficult to produce a uniform hardness of, and to polish uniformly the surface of a rubber sheet that covers the platen roller 102.
Moreover, the large diameter platen roller 102 requires a large size of the thermal head 108, because it is necessary for a jut 108b not to touch to the surface of the platen roller 102. Thus, the distance from the thermal element 108a to the jut 108b is required to be large. This is an obstacle to a reduction of cost.
The color printer 200 of prior art has another problem.
In the color printer 200, there is no aforementioned problem, as the diameter of the platen roller 203 is smaller than the length of the print medium 201.
The cylindrical sheet guide 202 is stationary installed, and has a longer length of circumference than the length of the print medium 201. The surface of the sheet guide 202 is coated by a small friction material, such as Teflon (trade name), nevertheless, there occurs a friction between the print medium 201 and the sheet guide 202. This friction causes to change the transporting speed of the print medium 201, and generates a deviation of the print medium 201 from the correct position when an image is printed on the print medium 201. Especially, plural printing processes are required to get a color image, and the deviation of each process will differ one another. As a result, the image of each color that should be precisely positioned in a same place, is printed in different positions each other, thus the quality of color image is lost.
There is a further problem that is common to both the color printers 100 and 200. When the clamper 104 passes through between the thermal head 108 and the platen roller 102, the thermal head 108 should be lifted from the platen roller 102. This means that a driving mechanism is needed for lifting the thermal head 108, and the structure of the color printer 100 becomes complicated. This situation is the same with the color printer 200.